The present invention relates to an information input/output display device having information input and output capabilities, and more particularly to an information input/output display device which can deliver written information to an external device.
Hereinbelow, a liquid crystal device furnished with a display function and an information holding function will be exemplified and explained as an information input/output display device.
Heretofore, as an information input/output display device, there has been known a device which is made from a smectic liquid crystal or a ferroelectric substance, such as PLZT, possessing a display function and an information holding function.
For example, the liquid crystal devices known exploit various display principles based on nematic liquid crystals, cholesteric liquid crystals or smectic liquid crystals. Any of the devices displays information in such a way that the oriented state of liquid crystal molecules is changed by an external field such as an electric field, and that the resulting change of an optical property is utilized.
On the other hand, as methods for reading out information displayed on a display member to an external circuit, there has been proposed a system wherein the information is read out by detecting the change of a capacitance based on an external field such as an electric field as disclosed in U.S. patent application Ser. No. 521900 (corresponding to European Patent Application No. 83107895.1).
By way of example, a method for reading out information proposed in a paper "Light Pen and Readout Functions in Thermally Addressed Smectic Liquid Crystal Display" (Y. Nagae, et al, Japan Display '83, pp. 490-493) will be briefly explained (European Patent Applications Nos. 84101914.4 and 84105941.3).
FIG. 1 shows a plan view of a liquid crystal matrix panel. The liquid crystal matrix panel 1 is such that the interspaces between X electrodes 2a-2c and Y electrodes 3a-3c are filled up with a smectic liquid crystal (A phase).
Each part where the X electrode and the Y electrode intersect forms a picture element 4, the capacitance of which becomes C.sub.o =.epsilon..sub.o .multidot..epsilon..sub.s /d.multidot.S. In the preceding expression, .epsilon..sub.o =free space permittivity, .epsilon..sub.s =relative permittivity of the liquid crystal layer, d=distance between the X and Y electrodes, and S=area of the picture element.
Now, when a voltage V.sub.s whose voltage rise rate (dV/dt) is constant is applied to the Y electrode 3a, currents i.sub.s as shown in FIGS. 2(a) and 2(b) flow out from the X electrodes. Since, on this occasion, the saturation value of the current i.sub.s becomes ##EQU1## information written in the liquid crystal matrix panel can be read out by discriminating the magnitude of the saturation value. This purpose can be achieved by connecting current-to-voltage converter circuits 5 to the X electrodes 2a-2c and comparing the output voltages of these circuits with a reference voltage V.sub.ref.
The aforementioned capacitance C.sub.o depends upon the distance d between the X and Y electrodes. On account of this fact, the disperson of d enlarges especially in a larger-sized panel, resulting in the large dispersion of the capacitance C.sub.o. Further, since the relative permittivity of the smectic liquid crystal fluctuates depending upon temperatures, the capacitance C.sub.o fluctuates depending also upon the temperatures.
With the prior-art method, therefore, errors occur in the read-out of information, making it difficult to construct a device of high reliability.